iwlegacy: fix BUG_ON(info->control.rates[0].idx < 0)

When trying to connect to 5GHz we can provide negative index to
mac80211 what trigger BUG_ON. Reason of iwl-3945-rs malfunction
on 5GHz is unknown and needs further investigation. For now, to
do not trigger a bug, correct value and just print WARNING.

Address bug:
https://bugzilla.redhat.com/show_bug.cgi?id=730653



Reported-and-tested-by: Jan Teichmann <jan.teichmann@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>